Membrane separation, which uses a selective membrane, is a fairly recent addition to the industrial separation technology for processing of liquid streams, such as water purification. In membrane separation, constituents of the influent typically pass through the membrane as a result of a driving force(s) in one effluent stream, thus leaving behind some portion of the original constituents in a second stream. Membrane separations commonly used for water purification or other liquid processing include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), electrodialysis, electrodeionization, pervaporation, membrane extraction, membrane distillation, membrane stripping, membrane aeration, and other processes. The driving force of the separation depends on the type of the membrane separation. Pressure-driven membrane filtration, also known as membrane filtration, includes microfiltration, ultrafiltration, nanofiltration and reverse osmosis, and uses pressure as a driving force, whereas the electrical driving force is used in electrodialysis and electrodeionization. Historically, membrane separation processes or systems were not considered cost effective for water treatment due to the adverse impacts that membrane scaling, membrane fouling, membrane degradation and the like had on the efficiency of removing solutes from aqueous water streams. However, advancements in technology have now made membrane separation a more commercially viable technology for treating aqueous feed streams suitable for use in industrial processes.
Furthermore, membrane separation processes have also been made more practical for industrial use, particularly for raw and wastewater purification. This has been achieved through the use of improved diagnostic tools or techniques for evaluating membrane separation performance. The performance of membrane separation, such as efficiency (e.g. flux or membrane permeability) and effectiveness (e.g. rejection or selectivity), are typically affected by various parameters concerning the operating conditions of the process. Therefore, it is desirable to monitor these and other types of process parameters specific to membrane separation to assess the performance of the process and/or the operating conditions. In this regard, a variety of different diagnostic techniques for monitoring membrane separation processes have been routinely used and are now understood and accepted as essential to its practicality and viability for industrial use.
However, monitoring is typically conducted on an intermittent basis, for example, once a work shift or at times less frequently. Known employed monitoring techniques can also be labor and time intensive. Thus, adjustments made to membrane separation processes in order to enhance performance based on typical monitoring may not be made in an expeditious manner. In addition, the presently available monitoring techniques often do not provide optimal sensitivity and selectivity with respect to monitoring a variety of process parameters that are generally relied on as indicators to evaluate and/or control membrane separation processes.
For example, monitoring techniques typically applied to reverse osmosis and nanofiltration include conductivity measurements and flow measurements. Conductivity measurements are inherently less accurate in order to determine the recovery of solutes which are substantially retained by the membrane. In this regard, conductive salts, typically used as indicators during conductive measurements, can pass through the membrane. Since salts generally pass through the membrane as a percentage of the total salt concentration, changes in local concentration due to concentration gradients or the like can change the conductivity of the product water without necessarily indicating membrane damage. This is especially true in the last stage of a multi-stage cross flow membrane system where salt concentrations (and, therefore, passage of salts as a percentage of that concentration) reach their highest levels. In this regard, the salt passage/percent rejection parameter is generally determined as an average value based on values measured during all stages of the membrane system.
Further, flow meters generally employed in such systems are subject to calibration inaccuracies, thus requiring frequent calibration. Moreover, typical monitoring of reverse osmosis and other membrane separations can routinely require the additional and/or combined use of a number of different techniques, thus increasing the complexity and expense of monitoring.
Accordingly, a need exists to monitor and/or control membrane separation processes which can treat feed streams, such as aqueous feed streams, suitable for use in industrial processes where conventional monitoring techniques are generally complex and/or may lack the sensitivity and selectivity necessary to adequately monitor one or more process parameters specific to membrane separation processes which are important to the evaluation of the performance of membrane separation.